The present invention generally relates to laser-based systems useful in construction applications and, more particularly, to a laser-based system for measuring the position of a receiver, mounted on mobile construction equipment operating at a work site, with respect to a transmitter located at a stationary reference position at the work site. The position of the receiver is determined in three dimensions by the system.
In prior laser-based systems, such as that disclosed in U.S. Pat. No. 3,588,249, for example, a reference plane is established throughout a work site by a transmitter which emits laser energy in a level reference plane. The reference plane is typically established by a laser beam which is projected radially outward from the transmitter and rotated continuously through 360 degrees to sweep around the entire work site. One or more receivers may be employed throughout the work site to sense the location of this reference plane. Such receivers may be mounted on a surveyor rod as described in U.S. Pat. No. 4,030,832, or they may be employed as part of a control system for construction or agricultural equipment as described in U.S. Pat. Nos. 3,813,171; 3,873,226; 3,997,071; and 4,034,490.
Prior laser systems have provided an indication of elevation throughout the work site, but they have not tyPically indicated the position of the receiver within the work site. Thus, for example, the height of the blade on a road grader with respect to the reference plane can be measured by such prior systems, but if this height is to vary over the work site it is necessary for the operator to determine by some other means where the road grader is located within the site in order to know precisely what the elevation should be at that location. In other words, prior laser systems provide only one dimension of position information, that being elevation.
The laser survey system disclosed in U.S. Pat. No. 4,830,489 provides not only elevation information, but also position information in two other axes. The system includes a laser transmitter, located at a reference position at a work site, which sweeps a laser beam radially in a reference plane. The system includes a receiver, located on mobile earthmoving equipment operating at the work site, which has a sensor that determines the relative elevation of the laser reference plane. The receiver also includes a pair of reflectors, each of which reflects laser energy back to the transmitter. The laser transmitter has a sensor which receives the reflected laser energy, and, in response thereto, produces receiver position information for transmission to the receiver.
The laser transmitter is designed to rotate the laser beam continuously through 360 degrees at a substantially constant angular velocity and thus sweep the beam past the two reflectors of the receiver once during each revolution. During each revolution of the laser beam, the transmitter receives back two short bursts or pulses of laser energy from the two reflectors. Thus, since the laser beam sweeps at a substantially constant angular velocity and the distance between the reflectors is fixed, the time period between receipt of these two pulses provides an accurate basis for the calculation of the range, or distance of the receiver from the transmitter. Since the accuracy of the range calculation is dependent upon a uniform rotational velocity for the laser beam, any variability in the rotational velocity will decrease the accuracy of the range calculation.
The laser survey system disclosed in the above cross-referenced patent application likewise provides not only elevation information, but also position information in two other axes. The system includes a laser transmitter located at a reference position at a work site, which sweeps a laser beam radially in a reference plane. The system includes a receiver, located on mobile earth moving equipment operating at the work site, which has a sensor that determines the relative elevation of the laser reference plane. The receiver also includes a pair of vertically-oriented retroreflectors, at least one of which will reflect laser energy back to the transmitter. The laser transmitter has a sensor for sensing laser beam energy reflected back to the transmitter from the receiver, a timer for momentarily shutting off the laser beam after the reflected laser beam has been sensed and a counter for producing an electrical signal related to the time period it takes the laser beam to travel from the transmitter to one of the retroreflectors and then back to the transmitter. The transmitter takes this time period, which is directly proportional to the range of the receiver from the transmitter, and produces receiver position information for transmission to the receiver. The position angle of the receiver with respect to a reference axis is also determined by employing a shaft angle encoder. The encoder is coupled to rotate with the laser beam so that the position angle of the receiver may be read from the encoder when a reflected light pulse is received by the transmitter from the receiver.
It is apparent that the transmitter must receive and sense reflected laser energy from the receiver in order to determine both the range of the receiver from the transmitter and the angular position of the receiver from the reference axis. Since the transmitter must respond to reflected laser energy in order to determine the range and angular position of the receiver, the number of receivers which can be serviced by a transmitter is limited.
While the noted three dimensional measurement systems are substantial improvements over the prior art, alternative approaches for determining both the range of the receiver from the transmitter and the angular position of the receiver are needed to advance the state of the art and to offer additional options to those working in the construction industry. An approach is desired in which both the range and the angular position of the receiver may be determined without the transmitter having to receive and respond to laser energy reflected from the receiver.